This invention relates to an automatic control system for a textile tenter frame apparatus.
In the process of finishing textile goods which are knit either completely or partially from synthetic materials, the goods are processed through a tenter frame where they are raised to a temperature sufficiently high to cause the material to be in a plastic state. The tenter hooks or pins on the frame fix the mechanical dimensions of the material and the process thus "sets" the material to that dimension. Since textile goods are sold by the lineal yard with a guaranteed weight per lineal yard, called yield, and since variations in yield of the knit fabric typically occur at the knitting machines due to variations in yarn weights and knitting conditions, an overfeed roll is normally employed at the entrance to the tenter frame. This overfeed roll is run at either a faster or slower rate than the tenter frame speed to produce finished yields that are either higher or lower than the yields resulting from the knitting operation. Heretofore, the operator made periodic adjustments to the overfeed roll speed based on his experience with the particular fabric being run and from finished yields actually obtained at the exit of the tenter frame.
Additional problems in the fabric finishing operation which are related to the wide variations that exist in the knitting operations included the relationship of the length of the knit pieces fabric received from the knitting machines compared to the length of the tenter frame and the variations of moisture levels in the knit goods as they enter the tenter frame. Pieces of knit goods are generally made in lengths of 40 to 50 yards due to mechanical limitations of the knitting machines producing the goods. These mechanical limitations are generally storage capacity limitations on the knitting machines themselves and presently attainable knitting speeds. The usual high production rates required in fabric production preclude use of a single knitting machine to produce an ordered style. Consequently, to provide a more continuous tentering operation, pieces of a given style which have been produced by a number of knitting machines are sewn together and fed continuously to the tenter frame. Further, because of the short style runs which are generally 20 to 30 pieces, styles are run contiguously by sewing the first piece of one style to the last piece of its predecessor. The aforesaid procedure created another problem in fabric processing.
The length of a typical tenter frame is in the order of 25 to 30 yards. Therefore, by the time a piece of fabric goods begins its exit from the tenter frame, at least one half of the piece is in the frame itself. Thus, any subsequent control moves to the overfeed roll based on exit fabric yield will affect, at best, one half of the piece being finished. Control moves based on the yield of the second half of a particular piece, based on its exit yield as it leaves the tenter frame, will be made on the next following piece whose entrance (as knitted) yield does not necessarily have a relationship to the entrance yield of the preceding pieces.
Prior to the present invention, control of the yield was attempted by mechanically determining the yield of finished pieces using a scale and length accumulator and notifying the operator at the entrance to the tenter frame of deviations from target yield. He would then make adjustments to the overfeed drive control based on the observed error. Since, as described above, variations in fabric yield as knitted are not necessarily related within a given style, these operator changes to overfeed speed were often incorrect for the piece of fabric then being finished.
Subsequent attempts to improve yield control involved the placement of a weight measuring device at the exit of the tenter frame which measured specific weight (ounces per square yard) of the fabric. This weight was then combined with the target exit width of the fabric and subsequent control moves were made to the overfeed drive control based on the calculated yield (ounces per lineal yard) of the fabric. As mentioned above, piece to piece variations in entry yield were not accounted for nor were variations in actual exit width as compared to target exit width.
In an attempt to improve the above system another weight measuring device, in addition to the weight measuring device at the tenter frame exit, was installed at the entrance to the tenter frame between the overfeed roll and the mechanical water removal device (if one existed). Based on an assumed water content, the entry weight was calculated and feed forward control moves were made to the overfeed roll in addition to the feedback control from the exit weight measuring systems. However, this method suffered from two major shortcomings. First, as mentioned above, substantial variations in moisture at the entry to the tenter frame were not accounted for but were interpreted as entry yield variations, giving rise to inaccurate control moves. Secondly, wide variations in fabric width at the entry to the tenter frame (prior to securing the fabric to the tenter hooks or pins) were experienced. Thus, a variation in width causing subsequent variations in specific weight at the entry to the tenter frame were interpreted as variations in entry yield, thereby giving rise to inaccurate control moves.